I. Field of the Invention
This invention relates to the field of communications systems and, in particular, to the field of transmission of message signals in a communications system.
II. Background of Invention
It is well known in the art of cellular communications systems to mix message signals to be transmitted with spreading code vectors such as Walsh code vectors. This permits the message signals to be combined, transmitted, and then separated from each other at the receiver after transmission. It is possible to separate the received signals because the spreading code vectors are orthogonal and they provide a theoretical interference of zero between the signals that are combined.
In order to perform these operations it is known to randomly assign one of the available spreading codes to each new originating call or each new handoff call added to the communications system. However, random assignment of spreading codes in this manner may result in large peaks in the transmit power level of the combined signals.
A serious consequence of the power level peaks is that the power amplifier that amplifies the combined signals can be temporarily driven into a nonlinear region and saturated, thus, distorting the input signal. This can cause interference between the combined signals, particularly between signals on adjacent channels. The interference between the combined signals can cause degradation of the separated and recovered signals.
This problem can be solved by providing a power amplifier with an increased capacity. Such a power amplifier is not driven into its nonlinear region by the peaks in the power level of the combined signals. However, this is an expensive and inefficient solution to the problem because the increased capacity of the power amplifier is not used during the remaining ninety-nine percent of the time.
Thus, it is desirable to provide a system and method for smoothing the transmit power level of the combined signals caused by random assignment of spreading codes in order to cause fewer peaks and drive the power amplifier into its nonlinear region less frequently.